The present invention relates to a rotational angle detecting sensor incorporated in construction machines or industrial machines, especially to a sensor to which high loads are applied.
Such rotational angle detecting sensors include ones of which the rotary shaft has one end thereof received in a housing and rotatably supported so that the rotary shaft can rotate with high accuracy (for example, see Japanese utility model publication 2604100).
FIGS. 2A and 2B show such a rotational angle detecting sensor. The rotational angle detecting sensor has a cylindrical housing 51 having an open end and an end wall 51a closing the other end thereof. The sensor further includes a rotary shaft 52 extending through the end wall 51a and having a large-diameter portion 52a at one end thereof. The large-diameter portion is received in the housing 51 and rotatably supported by two ball bearings 53 preloaded in a stationary position. The housing has its open end covered by a covering member 54. A rotational angle detector 55 is disposed between the inner surface of the covering member 54 and the end surface of the large-diameter portion 52a. The rotational angle detector 55 includes a ring-shaped detecting substrate 56 fitted around a circular protrusion 54a formed centrally on the inner surface of the covering member 54, a disc-shaped substrate 57 screwed to the end surface of the large-diameter portion 52a, and a known rotational angle detecting mechanism disposed between the opposed surfaces of the substrates 56 and 57.
As shown in FIG. 2B, an arcuate groove 54b is formed in the circular protrusion 54a of the covering member 54 concentrically with the rotary shaft 52. A stopper pin 58 extends through the substrate 57 and is received in the groove 54b so as to be movable in the groove 54b. The stopper pin 58 is adapted to abut against the ends of the groove 54b, thereby limiting the rotational range of the rotary shaft 52.
Another conventional means for limiting the rotational range of the rotary shaft 52 includes a semicircular protrusion 54c formed on the inner surface of the covering member 54 so that the stopper pin 58 is movable in the space defined between the protrusion 54c and the inner surface of the ring-shaped substrate 56. The movement of the stopper pin 58 is limited by the flat wall of the protrusion 54c. 
In these arrangements, if a large force is applied to the stopper pin with the movement of the pin restricted by the protrusion, the pin may be broken into pieces under stress greater than the strength of the pin. The pieces may damage the rotational angle detector. This increases the cost for replacement of the detector.
One way to prevent this problem would be to reduce the distance between the fixed position of the stopper pin and the position where the stopper pin contacts the protrusion, thereby reducing the stress applied to the stopper pin. However, because the stopper pin extends through the detecting substrate and is fixed to the large-diameter portion of the rotary shaft substrate in order to reduce rattling of the substrate, which is screwed to the large-diameter portion of the rotary shaft, its length has to be greater than the thickness of the substrate. Thus, the stress applied to the pin can be reduced only to a certain extent, so that it is impossible to reliably prevent breakage of the stopper pin and thus damage to the rotational angle detector.
If in the arrangement of FIG. 3, it is desired to increase the rotational range of the rotary shaft to more than 180 degrees, the protrusion has to be formed into a fan shape. This makes it difficult to fit the detecting substrate on the protrusion with high positional accuracy, which in turn makes it difficult to detect the rotational angle with high accuracy.
An object of the present invention is to provide a rotational angle detecting sensor of which the rotational detecting angle detector is less likely to be damaged.